In zirconium oxide (IV) (ZrO2) (referred to as “zirconia” hereinafter), there are polymorphs, and zirconia is subject to phase transition between the polymorphs. Tetragonal zirconia changes to monoclinic zirconia by the phase transition, for example. Therefore, since the phase transition leads to destruction of the crystal structure even though a sintered body is made of only zirconia, there is a problem that the sintered body of only zirconia does not have enough strength for a product. There is also another problem that a size of the sintered body of only zirconia is changed by volume change caused by the phase transition.
Therefore, stabilized zirconia and partially-stabilized zirconia (PSZ) have been used, in which the phase transition is inhibited by adding oxide, such as calcium oxide, magnesium oxide, yttrium oxide and cerium oxide, as a stabilizing agent to zirconia. In particular, the partially-stabilized zirconia is a ceramic having an outstanding property such as high strength and high toughness, and a sintered body of the partially-stabilized zirconia has been used in various ways such as a prosthetic material for dental treatment, a tool, and the like.
However, a problem of lack of long-term stabilization has not been solved yet because the partially-stabilized zirconia is nothing but a partially stabilized body. When the partially-stabilized zirconia sintered body is heated to 200 degrees Celsius under a presence of water, for example, the phase transition from the tetragonal system to the monoclinic system occurs, and this degrades the strength of the partially-stabilized zirconia sintered body (referred to as “low-temperature degradation” hereinafter). Accordingly, an art for manufacturing the zirconia sintered body has been developed to inhibit the low-temperature degradation (see Patent Literature 1 to Patent Literature 5, for example).
In the background arts according to Patent Literature 1 and Patent Literature 2, minute powder of the partially-stabilized zirconia having an average particle size of 0.5 μm or less is sintered at a temperature of 1200-1400 degrees Celsius to manufacture the zirconia sintered body.
In background arts according to Patent Literature 3 and Non-Patent Literature 1, in order to obtain a zirconia sintered body in which the low-temperature degradation phenomenon does not occur, the zirconia sintered body is manufactured by applying a solution including a compound such as Y to a surface of a unburned compact of a zirconia material including Y2O3 and the like and then burning it at a temperature of 1300-1800 degrees Celsius.
A zirconia sintered body disclosed in Patent Literature 4 is a zirconia sintered body including ZrO2, a rare earth metal oxide (R2O3) such as Y2O3 and the like, a boron compound, SiO2 and Al2O3, wherein a molar ratio of Al2O3 to a ingredient (M) including ZrO2, the rare earth metal oxide (R2O3), the boron compound and SiO2 (Al2O3/M) is 10/90-50/50, and a molar ratio of the rare earth metal oxide (R2O3) to ZrO2(R2O3/ZrO2) is 1/99-6/94, and crystal particles of ZrO2 are mainly formed from a phase of a tetragonal system or a mixed phase of the tetragonal system and a cubic system, a content of boron (B) to the total content of ZrO2 and the rare earth metal oxide (R2O3) is 0.05-2 mol %, and a content of SiO2 is 0.05-1.5 mol %. A zirconia material for medical use disclosed in Patent Literature 5 is based on ZrO2, it including oxide of rare earth metal such as Y2O3 and the like, a boron compound and Al2O3 and/or SiO2, a molar ratio of the oxide of the rare earth metal to ZrO2 being 1.5/98.5-5/95, a content of the boron compound being 0.05-8 mol % which is converted into a content of boron (B), a content of Al2O3 being 0.1-5 mol %, a content of SiO2 being 0.05-1.5 mol %.
Patent Literature 6 discloses a block for dental processing to manufacture a frame which is easy to grind or to grinding process after being completely sintered and which has flexural strength applicable to a bridge for multiple teeth loss. The block for the dental processing disclosed in Patent Literature 6 is a completely-sintered body of metal oxide having at least one of zirconia, alumina, mullite and spinel as a principle material and includes lanthanum phosphate and/or aluminum phosphate of 1 weight part to 23 weight parts in a crystal form to the metal oxide of 100 weight parts.    [PATENT LITERATURE 1] JP Patent Kokai Publication No. JP-P2001-80962A    [PATENT LITERATURE 2] JP Patent Kokai Publication No. JP-P2007-332026A    [PATENT LITERATURE 3] JP Patent Kokai Publication No. JP-A-03-115166    [PATENT LITERATURE 4] JP Patent Kokai Publication No. JP-A-07-215758    [PATENT LITERATURE 5] JP Patent Kokai Publication No. JP-A-08-33701    [PATENT LITERATURE 6] JP Patent Kokai Publication No. JP-P2009-23850A    [NON-PATENT LITERATURE 1] Taiji Yamamoto, Ichiro Kakei, “Improvement of heat resistance degradation by surface improvement of Y-TZP”, Zirconia ceramics 13 and 14, Uchida Rokakuho, 1998, pp. 147-163